A known problem with conventional optical transmission systems is the loss of power as an optical signal propagates through the transmission path between optical nodes. A well known solution to this problem involves dividing the transmission path into spans. Each span ends with an optical amplifier site (hereafter “amplifier”), which amplifies the signal in the optical domain, to compensate for the loss that occurred within the span. The amplifier also serves to couple adjacent spans, allowing the amplified signal to propagate through the next span.
Another problem is optical dispersion as the signal propagates through the optical fiber. Prior Art solutions compensate for the optical dispersion on a per span basis, by including an optical dispersion compensator as part of every span. The optical dispersion compensator typically forms part of the amplifier site, and typically comprises a segment of Dispersion Compensation Fiber (DCF). The length and characteristics of the segment of DCF are selected to have negative dispersion characteristics to those of the transmission span fiber, in order to offset the total dispersion along the span.
However, such an optical dispersion compensator introduces its own signal loss. Thus prior art dispersion compensation systems which compensate for dispersion on a per span basis, increase the total loss of each span, as loss of power occurs in both the transmission portion of the span, and in the DCF portion of the span. The loss of the DCF is not insignificant, being in the order of 8 dB of loss, whereas the loss for an 80 km transmission span having 0.25 dB/km loss is 20 dB. Thus the total loss is 28 dB.
A known solution is to include an Erbium Doped Fiber Amplifier (EDFA) or SOA (semiconductor optical amplifier) to amplify the signal to compensate for the loss. Typically a Two-staged EDFA is used, with the DCF segment between the two stages of the EDFA. The 1st stage is designed to have high gain in order to minimize the overall noise figure and the overall gain of the two stages is to compensate for total loss of the fiber span, including the DCF.
However, the EDFAs are expensive, and although an amplifier is needed for span loss, the inclusion of the DCF adds the additional cost of the Two-stage EDFA.
In order to avoid the cost of EDFAs, a known solution includes the use of Raman pump amplifiers. A Raman “pump” amplifies the signal by using a laser to inject another wavelength into the fiber, which acts to distributively amplify the signal. Typically, Raman pump amplifiers are at the junction between spans. There are two types of Raman pump. A “co-pump” is sent in the direction of transmission from the transmitter location. A counter pump is injected from a location in the opposite direction to that of transmission and has the effect of amplifying the signal which is received at that location.
As a signal often traverses multiple spans, it is important to compensate as much as possible for the total loss across each span, which includes the DCF loss. Thus the Raman Pump/amplifier should have sufficient power or gain to compensate for the total span loss. The additional loss introduced by the DCF has in the past sufficiently increased the cost and complexity of Raman pump amplifiers to the point where they are no longer cheaper than the Two-stage EDFAs. Thus, while showing great theoretical promise, an “All-Raman” system has proven to be very difficult to implement in a commercially viable manner.
Some systems use both an EDFA and a Raman pump (known as hybrid EDFA/Raman) to compensate for the total loss from the fiber and the DCF. However, although one EDFA is eliminated, a hybrid EDFA/Raman is not necessarily cheaper than a Two-stage EDFA.
It is, therefore, desirable to provide a Dispersion compensation method which does not add significant additional loss to the signal. By using such a technique, the Two-stage amplifier can be eliminated.